Solids, liquids and gases

The particle model pictures all matter as tiny particles. The state depends on how those particles are arranged, how they move, and how much energy they have:

• Solid — particles packed in a regular pattern, touching, vibrating about fixed positions. Fixed shape, fixed volume.
• Liquid — particles still touching but randomly arranged; they slide past one another. Fixed volume, takes the shape of its container.
• Gas — particles far apart, moving quickly in all directions, mostly empty space. No fixed shape or volume; can be compressed.

Energy ladder: heating gives particles more energy, so they move more and the forces holding them together are overcome: solid → liquid → gas.

Which state?

Changing state

Adding or removing energy moves a substance between states. These changes are physical — no new substance is made and they are reversible:

Physical, not chemical: ice, water and steam are all H₂O — the same particles, just rearranged. Melting and boiling are physical changes you can reverse; mass is conserved throughout.

Physical change vs chemical change

The particle model lets us tell two kinds of change apart:

• Physical change — particles stay the same substance; only their arrangement or spacing changes (e.g. melting, boiling, dissolving). Usually reversible.
• Chemical change — bonds break and re-form to make new substances (e.g. burning, rusting). Usually not easily reversible.

In both kinds of change, no particles are created or destroyed — so mass is conserved.

Watch out: a change being hard to reverse doesn't make it chemical, and "a gas was given off" doesn't always mean a reaction — boiling water releases steam with no new substance.

Physical or chemical?

Tap whether each change is physical or chemical.

Limits of the simple model

The simple particle model draws each particle as a tiny solid sphere — like a bowling ball. That's useful, but it is a model, so it has limitations:

• It ignores the forces of attraction between particles (which is why it can't explain why different materials melt and boil at different temperatures).
• It treats particles as having no internal structure and being solid, when in fact atoms are mostly empty space.
• It assumes the spheres are all the same size and ignores the real space between them.

HT exam tip: if asked for limitations, mention that the model takes no account of the forces between particles, the size of the particles, or the space between them.

Spotting the weakness

Inside the atom

Every atom has a tiny central nucleus made of protons and neutrons, surrounded by electrons in shells. Almost all the mass sits in the nucleus, but the nucleus is far smaller than the whole atom:

Scale: a typical atom has a radius of about 1 × 10⁻¹⁰ m (0.1 nm). The nucleus is thousands of times smaller still — so an atom is mostly empty space.

Charges and masses

You need the relative charge and relative mass of each sub-atomic particle:

An atom has no overall charge because it has equal numbers of protons (+1) and electrons (−1). Lose or gain electrons and it becomes a charged ion.

Atomic number & mass number

Two numbers describe an atom's nucleus, written in the standard notation:

• Atomic number (Z) = number of protons (and, in a neutral atom, electrons). It defines the element.
• Mass number (A) = number of protons + neutrons.
• So number of neutrons = A − Z (mass number − atomic number).

Your turn — counting neutrons

Isotopes

Isotopes are atoms of the same element (same number of protons, so same atomic number) but with different numbers of neutrons — so a different mass number.

Watch out: isotopes differ only in neutrons, never in protons. Changing the number of protons would change the element entirely. They have the same chemistry because they have the same electron arrangement.

Are they isotopes?

Relative atomic mass

Because an element is a mixture of isotopes, its relative atomic mass (A_r) is the weighted mean mass of its atoms, allowing for how common each isotope is (its abundance):

Watch out: A_r is a weighted average, not a simple one. It sits closer to the more abundant isotope — that's why chlorine's A_r (35.5) is nearer 35 than 37.

Your turn — relative atomic mass

Arranging the electrons

Electrons occupy shells (energy levels) around the nucleus, filling from the inside out. For the first 20 elements each shell holds:

• Shell 1 — up to 2 electrons
• Shell 2 — up to 8 electrons
• Shell 3 — up to 8 electrons (for the first 20 elements)

Write the arrangement as a comma list. The digits always add up to the number of electrons (= atomic number for a neutral atom):

Match the configuration

Tap an element, then tap its correct electron arrangement.

How the model changed

Scientists refined the model of the atom as new evidence appeared:

• Dalton — atoms are tiny solid spheres that can't be divided.
• Thomson — discovered the electron; proposed the "plum pudding" model (a ball of positive charge with electrons dotted in it).
• Rutherford (with Geiger & Marsden) — the alpha-scattering experiment showed most of the atom is empty space with a tiny, dense, positive nucleus.
• Bohr — electrons orbit the nucleus in fixed shells (energy levels).
• Later, the neutron was discovered, completing the nucleus.

The alpha-scattering experiment

Geiger and Marsden fired positive alpha particles at very thin gold foil. The results overturned the plum-pudding model:

Reading the evidence: straight-through = empty space; large deflection = something positively charged (repelling the positive alphas); bounce-back = that something is very dense and concentrated (the nucleus).

Build the timeline

Tap the models in the order they were developed, earliest first.

Why the model changed

Your turn — electron count

The C1 essentials

You've covered the whole of OCR Gateway C1 — Particles, both C1.1 the particle model and C1.2 atomic structure. Press Finish to see your score.